1. Field of the Invention
The present invention relates to an antiparallel (AP) pinned read sensor with an improved magnetoresistance and, more particularly, to an AP pinned structure which shunts less sense current IS.
2. Description of the Related Art
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position which is typically parallel to the ABS, is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum magnetoresistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve (magnetoresistive coefficient dr/R or readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos xcex8. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative magnetic fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve read head be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs the bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state, the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative magnetic fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a ferromagnetic coupling field HF between the pinned layer and the free layer, a demag field Hdemag from the pinned layer, and sense current fields HI from all conductive layers of the spin valve except the free layer.
When the sense current IS is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer toward a first direction perpendicular to the ABS. The pinned layer demagnetization field Hdemag further rotates the magnetic moment of the free layer toward the first direction counteracted by a ferromagnetic coupling field HF of the pinned layer that rotates the magnetic moment of the free layer toward a second direction antiparallel to the first direction.
Since the conductive material on the pinned layer side of the free layer far outweighs the conductive material on the other side of the free layer the sense current fields from the pinned layer side are a major force on the free layer which is difficult to counterbalance with the other magnetic forces acting on the free layer. Further, the conduction of the sense current IS through metallic layers of the spin valve sensor, other than the spacer layer, in effect shunts a portion of the sense current which reduces the amplitude of the signal detected by the read head. If less current is shunted through the conductive layers, other than the spacer layer, this can result in more sense current IS being conducted through the spacer layer to increase signal detection, or alternatively the sense current IS can be reduced to lower the generation of heat. If the pinned layer is an antiparallel (AP) pinned layer structure instead of a single pinned layer the aforementioned problems are exacerbated.
The AP pinned spin valve sensor differs from the simple spin valve sensor in that the AP pinned spin valve sensor has an AP pinned structure that has first and second AP pinned layers instead of a single pinned layer. An AP coupling layer is sandwiched between the first and second AP pinned layers. The first AP pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second AP pinned layer is immediately adjacent to the free layer and is antiparallel coupled to the first AP pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the AP coupling layer between the first and second AP pinned layers. Accordingly, the magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer.
The AP pinned structure is preferred over the single pinned layer because the magnetic moments of the first and second AP pinned layers of the AP pinned structure subtractively combine to provide a net magnetic moment that is less than the magnetic moment of the single pinned layer. The direction of the net moment is determined by the thicker of the first and second AP pinned layers. A reduced net magnetic moment equates to a reduced demagnetization (demag) field Hdemag from the AP pinned structure. Since the exchange coupling between the pinned and pinning layers is inversely proportional to the net pinning moment a reduced net magnetic moment increases the exchange coupling between the first AP pinned layer and the pinning layer. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin.
Since an AP pinned structure has more conductive metallic material on the pinned side of the free layer than a single pinned layer the sense current field on the free layer is increased from the pinned layer side of the free layer and more of the sense current is shunted instead of being conducted through the spacer layer. Accordingly, there is a strong felt need to provide an AP pinned structure as well as a single pinned layer that shunts less sense current for improving the biasing of the free layer and increasing signals detected by the read head.
A typical material employed for the simple pinned layer and the AP pinned layers of an AP pinned structure is cobalt (Co) or cobalt iron (CoFe). I have reduced sense current shunting through the single pinned layer or the AP pinned structure by providing the single pinned layer and one or both of the AP pinned layers of the AP pinned structure with a material that has a higher resistance than cobalt (Co) or cobalt iron (CoFe) while still maintaining desirable ferromagnetic properties. This has been accomplished by providing the single pinned layer or the pinned layer structure with a cobalt niobium (CoNb) based material that is amorphous. Cobalt niobium (CoNb), which is amorphous, has a resistance which is substantially five times the resistance of cobalt (Co), which is crystalline. It is important that sufficient niobium (Nb) be added to the cobalt (Co) to bring the alloy to an amorphous state. This percentage should be at least 5% and may be as high as 20%. It is desirable to keep the content of the niobium (Nb) as small as possible since niobium (Nb) reduces the magnetization (MS) of the pinned structure. In a preferred pinned structure a cobalt iron (CoFe) layer may be located between the cobalt niobium (CoNb) pinned layer and the spacer layer for promoting the magnetoresistive effect. However, if the pinned layer is cobalt iron niobium (CoFeNb) then the improved magnetoresistive effect may be obtained without the intervening cobalt iron (CoFe) layer. In the AP pinned structure only the first AP pinned layer may be provided with a cobalt niobium (CoNb) film and the second AP pinned layer maybe a cobalt iron (CoFe) layer. Alternatively, the cobalt niobium (CoNb) film may be employed in both of the AP pinned layers and in a preferred embodiment, may be sandwiched between cobalt iron (CoFe) films for improving texture of the films and increasing the magnetoresistive effect.
An object of the present invention is to provide a simple pinned layer or an AP pinned layer structure which shunts less sense current IS.
Another object is to provide a material for a pinned structure which, not only reduces sense current shunting because of a greater resistance to the sense current, but also has a composition which promotes magnetoresistance by interfacing the spacer layer.
A further object is to provide an AP pinned structure which employs a high magnetization low current shunting film in one or both of the AP pinned layers of the structure or is combined with cobalt (Co) or cobalt iron (CoFe) in one or both of the AP pinned layers for further increasing a magnetoresistance of the spin valve sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.